Water, wastewater and industrial liquid filtration units typically have an underdrain system for supporting filter media such that the filter media is spaced apart from the bottom of the filter. In addition to providing support for the filter media, the underdrain system serves two primary purposes: to collect the filtered water that passes through the media and to uniformly distribute backwash water, backwash air, or a combination of both, across the filter.
Underdrain systems were often made of precast concrete blocks. The high-strength concrete blocks are placed side-by-side and end-to-end in the bottom of the filter to provide a "false bottom." The blocks provide support for the filter media and are shaped to collect the filtered water that passes through the media. Additionally, the blocks have conduits, commonly known as laterals, which allow for the passage of air or clean water used as backwash. Piping, such as air headers, are part of the backwash air distribution system. Backwash water is brought into the system by flumes. Air headers bring in pressurized air which is distributed into the laterals by pipes from the top of the headers through the flumes and into the laterals or distribution conduits.
During backwash operations, air is forced through the air distribution pipes and into the air laterals of the underdrain blocks. The pressurized air can be used as a backwash or combined with water for an air/water backwash. The pressurized air/water combination causes the filtered water to be passed upward through the media with sufficient velocity to prevent filter problems such as mud balls, filter cracking, agglomeration buildup on the media grains, and inactive areas within the filter. Underdrain blocks also act to physically separate the filter media from the air distribution pipes to prevent clogging of the orifices in the pipes by the finer particles of the filter media.
One example of an underdrain system made of precast blocks is U.S. Pat. No. 4,923,606 to Gresh et al. The underdrain blocks disclosed in the '606 reference are a combination of a plastic jacket and a concrete mold. The underdrain blocks in the '606 reference are used for both upflow and downflow filtration units. Thus, the blocks provide distribution of liquid and air during backwash operations.
U.S. Pat. No. 5,160,614 to Brown discloses an underdrain system comprised of a series of modular, interconnected air duct blocks. In the '614 reference, the row of air duct blocks extend transversely to a plurality of laterals. The laterals are defined by the connection of individual underdrain blocks in parallel adjacent rows. The air duct blocks are interconnected such that the interiors of the separate air duct blocks are in fluid communication with one another to supply backwash gas through a single source of gas supply.
U.S. Pat. No. 5,108,627 to Berkebile et al. discloses an underdrain system comprised of blocks having a plurality of exterior and interior walls. The walls of the '627 reference define a plurality of interior chambers having at least a first and second conduit parallel to the longitudinal axis of the blocks. The interior walls defining the interior chambers provide bearing support for the top wall of the block and are disposed to provide even distribution of backwash flows which are facilitated by separate conduits within the interior chambers.
Because air/water backwash is superior to either backwash alone, there is a need for a new and improved underdrain system that allows both air laterals and water laterals during the liquid filter treatment. Existing shortened underdrain blocks that are capable of carrying both air and water backwash are limited to sixteen foot conduits because the area of the primary laterals is not large enough to carry large volumes of water, (the area of prior art shortened blocks is approximately 33 square inches.) Most existing underdrain blocks are twelve inches in height. Shorter blocks currently in use are limited in the volume of backwash air and water that they can carry.